Water resistant metal pigment particles

ABSTRACT

Metal pigment particles treated with an organic phosphate, phosphite or a pentavalent vanadium compound and a nitro-containing compound show increased stability against attack by water. The particles are especially useful in aqueous coating compositions.

BACKGROUND OF THE INVENTION

The present invention is related to paste compositions containing metalpigments suitable for forming coating compositions, particularly aqueouscoating systems. Increasingly stringent environmental regulations haverequired that coating systems dramatically reduce volatile organicsolvent levels. One way to comply with such regulations is to use waterin place of the volatile organic solvents previously used.

However, in the area of coating systems utilizing metal pigmentparticles, aqueous systems present rather formidable difficulties. Thisis particularly true with respect to aluminum and zinc pigments. Thus,the metal pigment can readily react with water to generate hydrogen gas.The amount of gas generated can produce a safety hazard, creating highpressures within the composition containers. Also, the water reactionsubstantially diminishes the aesthetic value of metal pigments. Thereaction of aluminum pigments with water can be depicted as follows:

    2Al+6H.sub.2 O→2Al(OH).sub.3 +3H.sub.2 (g)

Due to the increasing demand for aqueous systems, a number of techniqueshave been proposed for inhibiting the attack on the pigment particles bywater. Unfortunately, most of these techniques have not providedsufficient protection.

One technique that provides inhibiting properties is the passivation ofthe metal pigment particles with an ionic organic phosphate as disclosedby Williams et al., U.S. Pat. No. 4,565,716. Other techniques involvethe use of compounds containing pentavalent vanadium compounds asdisclosed in Kondis U.S. Pat. No. 4,693,754, or the use of organicphosphites as disclosed in Kondis et al. U.S. Pat. No. 4,808,231.Another composition of matter utilized to produce an aluminum paste foruse in aqueous systems uses nitroparaffin solvents to inhibit theevolution of hydrogen gas. However, through the addition ofnitro-containing solvents we have improved upon the passivation of metalpigment particles utilizing organic phosphates, phosphites, or vanadiumcompounds.

SUMMARY OF THE INVENTION

The present invention provides a composition of matter which comprisesmetal particles treated with either (a) an ionic organic phosphatecompound, for example as taught in Williams et al., U.S. Pat. No.4,565,716, or (b) a pentavalent vanadium compound, for example as taughtin Kondis, U.S. Pat. No. 4,693,754, or (c) an organic phosphite compoundas taught in Kondis et al. U S. Pat. No. 4,808,231, and anitro-containing solvent such as a nitroparaffin. The treated metalpigment particles can be used to form a pigment paste. The metalparticles. e.g. aluminum flakes, which have been treated with thephosphate, phosphite, or vanadate and nitro-containing compound arestabilized to a degree suitable for use in aqueous coating systems. Thecombination of the treatment with both the phosphate, phosphite, orvanadium compounds and the nitro-containing solvents produces a metalpigment paste which has improved gassing stability over the separatelytreated products.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As indicated above, the present invention is based on the discovery thata combination of certain types of organic phosphate compounds, organicphosphite compounds or pentavalent vanadium compounds and anitro-containing solvent effectively stabilizes metal pigments fromreacting with water, rendering the pigment suitable for use inwater-based coatings without significant degradation of opticalproperties. Either the phosphate compound, the phosphite compound, thevanadium compound or the nitro-containing solvent, used individually,provides some measure of stability for metal pigments in water-basedcoatings, but the combination of the nitro-containing solvent witheither the phosphate, the phosphite or the vanadate provides greatlyenhanced stability. The present invention is especially useful for zincand aluminum pigments.

The phosphate compound may be of the type described in Williams et al.,U.S. Pat. No. 4,565,716, the disclosure of which is incorporated hereinby reference. The terms "organic phosphate" and "phosphate" used hereinshould be understood to encompass materials having the followingformulas: ##STR1## wherein: X=1 or 2

Y=1 or 2 and

X+Y=3;

where Z is hydrogen or a means for neutralizing acid phosphate; andwhere each R group is independently benzo; a halo group; an alkyl grouphaving from 1 to 24 carbon atoms; a cycloalkyl group having from 3 to 20carbon atoms; an aryl group having from 6 to 40 carbon atoms; an alphaor beta naphthyl; a substituted alkyl, cycloalkyl, aryl, or naphthylgroup, as defined above, where said substituent is alkyl, cycloalkyl,aryl, or naphthyl; or a heterosubstituent substituted alkyl, cycloalkyl,aryl, or naphthyl group, as defined above, where said heterosubstituentsare selected from the group consisting of amino, hydroxy, epoxy,nitrilo, nitro, carboxy, carbonyl, pyridinyl, alpha or beta furyl, oralpha or beta thienyl. Representative examples of means for neutralizingacid phosphate are Na, Li, K, Zn, Mg, Ca, and NH_(L) (R')_(M), where

    L+M=3 or 4

    R'=C.sub.n H.sub.2n+1 or C.sub.n H.sub.2n OH

    and

    n=1 to 20

Preferred compounds are those of the type shown in Formula A, where theR groups each contain 8 or more carbon atoms, and Z is an amino group.Two especially preferred materials are marketed by Albright and WilsonAmericas, Inc., under the tradenames Virco-Pet® 40 and Virco-Pet® 50.The concentration of the phosphate compound should be from 1% to 25%,preferably 3% to 15%, based on metal pigment weight.

Organic phosphites are derivatives of phosphorous acid, rather thanphosphoric acid used to produce organic phosphates. These may or may notbe neutralized. The organic R groups and the inorganic Z groups are thesame as defined above. The organic phosphite may be of the typedescribed in Kondis et al. U.S. Pat. No. 4,808,231, the disclosure ofwhich is incorporated herein by reference. Preferred arealkyl-substituted phosphites; especially preferred dilauryl hydrogenphosphite, dioctyl hydrogen phosphite, and dioleyl hydrogen phosphite.The concentration of the phosphite compound should be 2% to 15%,preferably 5% to 10%, based on metal pigment weight.

The pentavalent vanadium compound may be vanadium pentoxide (V₂ O₅), aspecies containing an oxovanadium anion, a vanadium oxo halide, or avanadium alkoxide. Preferred are species containing an oxovanadiumanion, where the corresponding cation is an alkaline or alkaline earthmetal ion or

    NH.sub.L (R+).sub.M.sup.+ ;

    where

    L+M=4

    R'=C.sub.n H.sub.2n+1 or C.sub.n H.sub.2n OH

    and

    n=1 to 20

Examples of oxovanadium anions are VO₃ --, VO₄ ³⁻, V₂ O₇ ^(4"), V₃ O₈--, V₁₀ O₂₈ ⁶⁻, and various protonated forms of each. The concentrationof the pentavalent vanadium compound should be from 1% to 30%,preferably 5% to 15%, based on metal pigment weight.

The nitro-containing solvent may be a member of the alkyl or arylseries, or a halo- or amino-substituted alkyl or aryl, containing 1 to20 carbon atoms and a nitro (--NO₂) functional group. The lower membersof the nitroparaffin series, i.e., nitromethane, nitroethane and1-nitropropane, are preferred on the basis of toxicological propertiesand availability. The nitro-containing solvent should be present at 5%to 100%, but preferably 20% or more, most preferably 35% or more, of thetotal weight of solvent in the final metal pigment paste. The solvent isgenerally about 28% to 50% by weight of the paste.

The preferred method to incorporate the vanadate compound is a variationof the slurry method taught in Kondis U.S. Pat. No. 4,693,754, thedisclosure of which is incorporated herein by reference. A metal pigmentparticle filter cake or paste--typically containing 50% to 95%,preferably 60% to 85%, of metal pigment in mineral spirits solvent--isslurried in a mixture composed of 30% to 94.5%, preferably 65% to 89%,of the nitro-containing solvent; 5% to 40%, preferably 10% to 25%, of awater-miscible organic solvent such as an alcohol, glycol, glycol ether,or glycol ether acetate; and 0.5% to 30%, preferably 1% to 10%, ofwater, along with the desired amount of the vanadium compound. Theslurry is mixed vigorously at a temperature of 20° C. to 100° C.,preferably 50° C. to 80° C., for a period of time ranging from 0.5 to 72hours, preferably 5 to 48 hours; and then is filtered to obtain thedesired final non-volatile content, typically 60% to 72%.

One preferred method to incorporate the phosphate or phosphite compoundand the nitro-containing solvent with the aluminum pigment is referredto as "cake reduction," wherein a treatment solution of the phosphate orphosphite compound in the nitro-containing solvent is used to reduce, ordilute, a metal pigment particle filter cake to a paste. The filter cakeis typically 75% to 95%, preferably 78% to 85%, of metal pigment inmineral spirits solvent. The filter cake is charged into a mixer, andthe treatment solution is added, in a quantity sufficient to provideboth the phosphate or phosphite compound and the nitro-containingsolvent within the preferred ranges. Other organic solvents, as neededor desired, may be used. These include, but are not limited to, mineralspirits; high flash naphtha; alcohols; glycols; glycol ethers; glycolether acetates: toluene; xylene; and ketones. Water may also be used asa solvent, provided that a surfactant or a water-miscible solvent isalso used to provide a homogeneous product. The total amount of solventadded should be sufficient to obtain the desired final non-volatilecontent; typically, 60% to 72%.

The filter cake, treatment solution, and other solvents, if used, aremechanically mixed for a period of time from 5 minutes to 8 hours. Themixing time should be long enough to assure a uniform distribution ofthe treatment solution and the metal pigment particles, but not so longas to adversely affect the optical properties of the pigment.

Although the "cake reduction" method is preferred, other techniques maybe used to produce a water-stable metal pigment paste. In one method,the metal pigment filter cake is slurried in an excess of thenitro-containing solvent. The slurry is mechanically mixed for a periodof time from 5 minutes to 1 hour, then filtered to a non-volatilecontent of 75% to 95%, preferably 78% to 85%. This filter cake, in whichthe nitro-containing compound is essentially the only solvent, is thentreated with the phosphate or phosphite compound and additional solvent,either the nitrocontaining compound or other solvents, using the "cakereduction" method. A variation of the above method is to slurry thepigment filter cake in an excess of a solution of the phosphate orphosphite compound in the nitro-containing solvent. This slurry ismechanically mixed as described above, then filtered to the nonvolatilecontent desired for the final product, typically 60% to 72%. Both ofthese methods can be used to yield a product which is essentially freeof mineral spirits.

In addition to the post-milling treatment methods described above,another technique is to introduce the nitro-containing solvent and/orthe phosphate or phosphite compound into the ball mill, thus stabilizingthe metal pigment surfaces as they are being generated. If only one ofthe two ingredients is added to the ball mill, the other one can beincorporated using the "cake reduction" method.

The preferred treatment process provides excellent stability, regardlessof the lubricant used in milling, and does not alter the leafing ornon-leafing behavior of the pigment. The process also provides excellentstability using pigments of either regular or high purity metal.

The pigment paste obtained can be used in a variety of known coatingsystems, as a direct replacement for currently used pastes. Examplesinclude maintenance, general industrial, roof coating, and automotivecoating systems. Thus, the paste may be used, for example, with acrylicpolymer emulsions, water reducible alkyd resin systems, water reduciblealkyd/melamine cross-linked systems, waterborne epoxy coatings,polyester emulsions and water reducible polyester melamine coatings.

EXAMPLES Example 1

492 grams of a non-leafing aluminum pigment filter cake (79% aluminum inmineral spirits) is charged into a mixer. To this is added 61.9 grams ofVirco-Pet® 50 (70% water-insoluble phosphate ester in mineral spirits,from Albright and Wilson) and 96.1 grams of nitroethane, so that thenitroethane represents 44.1% of the total solvents. The material ismixed for one hour, then tested for aqueous stability as describedbelow.

Example 2

The aluminum pigment filter cake of Example 1 is slurried innitroethane, then filtered to a metal content of 78.5% to essentiallyreplace the mineral spirits with nitroethane. 489 grams of this washedfilter cake is charged into a mixer, then 61.9 grams of Virco-Pet® 50and 100 grams of nitroethane are added, so that the nitroethanerepresents 91.7% of the total solvents. The material is mixed for onehour, then tested for aqueous stability.

Comparative Example 1

492 grams of the aluminum pigment filter cake of Example 1 is chargedinto a mixer, and 61.9 grams of Virco-Pet® 50 and 96.1 grams of highflash naphtha are added. The material is mixed for one hour, then testedfor aqueous stability.

Comparative Example 2

488 grams of the aluminum pigment filter cake of Example 1 is chargedinto a mixer, and 103 grams of nitroethane is added, so that thenitroethane represents 50.1% of the total solvents. The material ismixed for one hour, then tested for aqueous stability.

Comparative Example 3

488 grams of the washed aluminum pigment filter cake of Example 2 ischarged into a mixer, and 103 grams of nitroethane is added, so that thenitroethane represents essentially 100% of the total solvent. Thematerial is mixed for one hour, then tested for aqueous stability.

The aluminum pigment pastes from the above examples were incorporatedinto a general industrial coating formulation, prepared as follows:

Enough of each paste to yield 20.2 grams of aluminum is weighed out.24.8 grams of glycol ether PM, 6.2 grams of glycol ether DPM, 4.7 gramsof Surfynol 104BC (Air Products), 61.9 grams of deionized water, and312.2 grams of 76 Resin 1018 (Unocal), a styrene-acrylate copolymeremulsion, is added, and the mixture blended until uniform.

The paints obtained are placed in a constant temperature bath at 52° C.,and the gas evolved is collected in an inverted water-filled buret for168 hours. The data are summarized in Table 1.

                  TABLE 1                                                         ______________________________________                                               VIRCO-    NITRO-      MLS. H.sub.2                                            PET ® 50                                                                            ETHANE      EVOLVED                                                 PERCENT OF                                                                              PERCENT OF  168 HRS.                                                ALUMINUM  SOLVENT     @ 52° C.                                  ______________________________________                                        EXAMPLE                                                                       NUMBER                                                                        1        16.13       44.1          3.4                                        2        16.13       91.7          6.4                                        COMPARA-                                                                      TIVE                                                                          EXAMPLE                                                                       NUMBER                                                                        1        16.13       0             11.5                                       2        0           50.1          38.8 (3 hrs.)                              3        0           100.0        >100 (19 hrs.)                              ______________________________________                                    

The data clearly show the synergy between the phosphate and thenitroethane. Nitroethane alone confers very little stability in thissystem, and the samples had to be removed very early in the test, due toexcessive gas generation. Virco-Pet® 50, alone, provides fairly lowgassing, but the results when it is combined with nitroethane are muchbetter.

Example 3

488 grams of the aluminum pigment filter cake of Example 1 is chargedinto a mixer. To this is added 48.1 grams of Virco-Pet® 40 (90%water-soluble phosphate ester in diethyl amine, from Albright andWilson), and 107 grams of nitroethane, so that the nitroethanerepresents 49.6% of the total solvents. The material is mixed for onehour, then tested for aqueous stability.

Example 4

489 grams of the washed aluminum pigment filter cake of Example 2 ischarged into a mixer, and 48.1 grams of Virco-Pet® 40 and 107 grams ofnitroethane is added, so that the nitroethane represents 97.8% of thetotal solvents. The material is mixed for one hour, then tested foraqueous stability.

Example 5

489 grams of the washed aluminum pigment filter cake of Example 2 ischarged into a mixer, and 14.5 grams of Virco-Pet® 40 and 141 grams ofnitroethane is added, so that the nitroethane represents 99.4% of thetotal solvent. The material is mixed for one hour, then tested foraqueous stability.

Comparative Example 4

488 grams of the aluminum pigment filter cake of Example 1 is chargedinto a mixer, and 48.1 grams of Virco-Pet® 40 and 107 grams of highflash naphtha is added. The material is mixed for one hour, then testedfor aqueous stability.

The aluminum pigment pastes from the above examples were incorporatedinto the general industrial coating formulation described above, andtested for gas evolution The data are summarized in Table 2.

                  TABLE 2                                                         ______________________________________                                               VIRCO-    NITRO-      MLS. H.sub.2                                            PET ® 40                                                                            ETHANE      EVOLVED                                                 PERCENT OF                                                                              PERCENT OF  168 HRS.                                                ALUMINUM  SOLVENT     @ 52° C.                                  ______________________________________                                        EXAMPLE                                                                       NUMBER                                                                        3        12.53       49.6          54.2                                       4        12.53       97.8          16.9                                       5        3.76        99.4          25.5                                       COMPARA-                                                                      TIVE                                                                          EXAMPLE                                                                       NUMBER                                                                        4        12.53       0           >100.0 (48 hrs.)                             ______________________________________                                    

Again, the results, when both nitroethane and the phosphate are present,are much better than for either one alone. It can t also be seen that,in this paint formulation, Virco-Pet® 40 is not as effective asVirco-Pet® 50.

The aluminum pigment pastes from Examples 1, 2, 4 and 5, and fromComparative Examples 1 and 2, were incorporated into an automotivebasecoat formulation, prepared as follows:

Enough of each paste to yield 3.56 grams of aluminum is dispersed in10.52 grams of glycol ether EB and blended until uniform. To thisdispersion is added 419.76 grams of a pigmented polyester/polyurethaneautomotive aqueous basecoat formulation, and the mixture is blendeduntil uniform. These paints are then tested for gas evolution. The dataare summarized in Table 3.

                  TABLE 3                                                         ______________________________________                                                          NITRO-                                                      PHOSPHATE ESTER     ETHANE    MLS. H.sub.2                                                PERCENT     PERCENT   EVOLVED                                                 OF          OF        168 HRS.                                    TYPE        ALUMINUM    SOLVENT   @ 52° C.                             ______________________________________                                        EX-                                                                           AMPLE                                                                         NUM-                                                                          BER                                                                           1      Virco-   16.13       44.1    26.1                                             Pet ® 50                                                           2      Virco-   16.13       91.7    13.8                                             Pet ® 50                                                           4      Virco-   12.53       97.8    14.3                                             Pet ® 40                                                           5      Virco-   3.76        99.4    12.6                                             Pet ® 40                                                           COM-                                                                          PARA-                                                                         TIVE                                                                          EX-                                                                           AMPLE                                                                         NUM-                                                                          BER                                                                           1      Virco-   16.13       0       60.0                                             Pet ® 50                                                           2      NONE     0           50.1    72.8                                      ______________________________________                                    

The results in the automotive basecoat formulation also show a largereduction in gassing when using both nitroethane and the phosphate ascompared to using either, individually. Increasing the nitroethanecontent provides an additional reduction in gassing, whether Virco-Pet®50 or Virco-Pet® 40 is used.

Example 6

749.3 grams of a leafing aluminum pigment paste; HYDRO PASTE® 830 WATERDISPERSIBLE ALUMINUM PIGMENT from Silberline, 65% aluminum in a 5/4blend of mineral spirits/1-nitropropane, is charged into a mixer. 54.0grams of Virco-Pet® 50 is added, and the material is mixed for one hour.It is then tested for aqueous stability and appearance as describedbelow.

Example 7

543.4 grams of a leafing aluminum pigment filter cake; 82.8% aluminum inmineral spirits; is charged into a mixer. 45.0 grams of Virco-Pet® 50and 78.8 grams of nitroethane is added, so that the nitroethanerepresents 42.4% of the total solvents. The material is mixed for onehour, then tested for aqueous stability and appearance.

Comparative Example 5

The HYDRO PASTE® 830 WATER DISPERSIBLE ALUMINUM PIGMENT of Example 6 istested for aqueous stability and appearance.

Comparative Example 6

543.4 grams of the aluminum pigment filter cake of Example 7 is chargedinto a mixer, and 125.0 grams of nitroethane is added, so that thenitroethane represents 57.2% of the total solvents. The material ismixed for one hour, then tested for aqueous stability and appearance.

Comparative Example 7

543.4 grams of the leafing aluminum pigment filter cake of Example 7 ischarged into a mixer, and 45.0 grams of Virco-Pet® 50 and 78.8 grams ofhigh flash naphtha is added. The material is mixed for one hour, thentested for aqueous stability and appearance.

96.1 grams of the aluminum pigment pastes from each of the aboveexamples were incorporated into 400.0 grams of a commercially availablewaterborne asphalt roof coating vehicle. A portion of each was drawndown on laminated cardboard, using a Bird applicator, and thereflectance was measured using a Total Reflectometer. The remainder wastested for gas evolution. The data are summarized in Table 4:

                  TABLE 4                                                         ______________________________________                                        VIRCO-       NITRO-                                                           PET ® 50 PARAFFIN    MLS. H.sub.2                                         % OF                 % OF    EVOLVED  RE-                                     ALUMI-               SOL-    168 HRS. FLEC-                                   NUM          TYPE    VENT    @ 52° C.                                                                        TANCE                                   ______________________________________                                        EX-                                                                           AMPLE                                                                         NUM-                                                                          BER                                                                           6      11.1      1-nitro-                                                                              41.2  6.4      41.1                                                   pro-                                                                          pane                                                         7      10.0      nitro-  42.4  5.7      53.6                                                   ethane                                                       COM-                                                                          PARA-                                                                         TIVE                                                                          EX-                                                                           AMPLE                                                                         NUM-                                                                          BER                                                                           5      0         1-nitro-                                                                              44.4  17.8     44.3                                                   pro-                                                                          pane                                                         6      0         nitro-  57.2  20.3     55.4                                                   ethane                                                       7      10.0      NONE    0     19.6     46.8                                  ______________________________________                                    

Again, the combination of either nitroethane or 1-nitropropane withVirco-Pet® 50 results in reduced gas evoluation compared to using any ofthe three ingredients, alone. Aesthetically, nitroethane gives higherreflectance values than 1-nitropropane, and the combination ofVirco-Pet® 50 with either solvent lowers the reflectance. However, themixture of nitroethane and Virco-Pet® 50 has a reflectance greater thaneither 1-nitropropane or Virco-Pet® 50, alone.

Example 8

126.6 grams of the aluminum pigment filter cake of Example 1 is chargedinto a two-liter reaction flask and slurried in 1000.0 grams of solventcontaining 81.3% nitroethane, 16.3% glycol ether DE acetate, and 2.4%water. 10.0 grams of ammonium metavanadate (NH₄ VO₃) were then added,and the slurry was stirred magnetically for 24 hours at 70° C.. Thematerial was then filtered to approximately 65% non-volatile content,then tested for aqueous stability.

Comparative Example 8

The reaction of Example 8 was repeated, except that the slurry solventwas composed of 80% glycol ether DE acetate and 20% water. The filteredmaterial was then tested for aqueous stability.

The aluminum pigment pastes from the above examples and from ComparativeExample 2 were incorporated into the pigmented automotive basecoatformulation described above, and tested for gas evolution. The data aresummarized in Table 5:

                  TABLE 5                                                         ______________________________________                                                                      MLS. H.sub.2                                                                  EVOLVED                                         VANADATE                      168 HRS.                                        TREATED         SOLVENTS      @ 52° C.                                 ______________________________________                                        EX-                                                                           AMPLE                                                                         NUMBER                                                                        8       YES         Nitroethane,   4.1                                                            DE Acetate, Water,                                                            Mineral Spirits                                           COM-                                                                          PARA-                                                                         TIVE                                                                          EX-                                                                           AMPLE                                                                         NUMBER                                                                        8       YES         DE Acetate, Water,                                                                          34.0                                                            Mineral Spirits                                           2       NO          Nitroethane,  72.8                                                            Mineral Spirits                                           ______________________________________                                    

The data show that the synergistic effect of nitroethane occurs withvanadates, as well as with phosphates.

Although a detailed description of the invention with specific exampleshas been provided above, the present invention is not limited thereto,but rather is defined in the following claims.

What is claimed is:
 1. A paste suitable for formation of a coatingcomposition, comprising:metal pigment particles; a member selected fromthe group consisting of an organic phosphate, an organic phosphite and apentavalent vanadium compound; and a solvent which comprises an alkyl oraryl compound having 1 to 20 carbon atoms or a halo or amino substitutedalkyl or aryl compound having 1 to 20 carbon atoms and wherein saidcompound contains a nitro a nitro functional group.
 2. The paste ofclaim 1, wherein the metal pigment particles are made of aluminum orzinc.
 3. The paste of claim 1, wherein the metal pigment particles arepresent in an amount of about 60-72% by weight.
 4. The paste of claim 1,wherein an organic phosphate is present in an amount of about 1 to 25%by weight of the metal pigment particles.
 5. The paste of claim 1,wherein the solvent is present in an amount of about 28 to 40% by weightof the paste and the nitro-containing compound is present in an amountof about 5 to 100% by weight of the solvent.
 6. The paste of claim 4,wherein the amount of organic phosphate is about 3 to 15% by weight ofthe metal pigment particles.
 7. The paste of claim 5, wherein the amountof nitro-containing compound is about 20 to 100% by weight of thesolvent.
 8. The paste of claim 7, wherein the amount of nitro-containingcompound is about 35 to 100% by weight of the solvent.
 9. The paste ofclaim 1, wherein a pentavalent vanadium compound is present in an amountof about 1 to 30% by weight of the metal pigment particles.
 10. Anaqueous coating composition comprising:a metal pigment-containing pasteas claimed in claim 1; and an aqueous carrier.
 11. The paste of claim 9,wherein the amount of the pentavalent vanadium compound is about 5 to15% by weight of the metal pigment particles.
 12. The paste of claim 1,wherein an organic phosphite is present in an amount of about 2 to 15%by weight of the metal pigment particles.
 13. The paste of claim 12,wherein the amount of the organic phosphite is about 5 to 10% by weightof the metal pigment particles.
 14. A method of making a metal pigmentpaste useful for forming a coating composition, comprising:producingmetal pigment particles; and treating the pigment particles with (a) anorganic phosphate, (b) an organic phosphite or (c) a pentavalentvanadium compound, and a solvent which comprises an alkyl or arylcompound having 1 to 20 carbon atoms or a halo or amino substitutedalkyl or aryl compound having 1 to 20 carbon atoms and wherein saidcompound contains a nitro a nitro functional group.
 15. The method ofclaim 14, wherein the pigment particles are treated with a treatmentsolution which comprises the organic phosphate, organic phosphite orpentavalent vanadium compound and the solvent.
 16. The method of claim14, wherein the pigment particles are first treated with the solvent andthen treated with an organic phosphate or organic phosphite.